Mixed-mode digital predistortion

ABSTRACT

Various examples are directed to systems and methods for digital predistortion (DPD). A linear digital predistortion (DPD) circuit may be programmed to generate a pre-distorted signal linear component based at least in part on a complex baseband signal. A nonlinear DPD circuit may be programmed to generate a pre-distorted signal nonlinear component based at least in part on the complex baseband signal. A mixer circuit programmed to generate a pre-distorted signal based at least in part on the pre-distorted signal linear component and the pre-distorted signal nonlinear component.

TECHNICAL FIELD

This document pertains generally, but not by way of limitation, tointegrated circuits and communication systems, and particularly, but notby way of limitation to digital predistortion for power amplifiers.

BACKGROUND

Radiofrequency (RF) communications, such as for mobile telephony, mayuse an RF power amplifier (PA) circuit in an RF transmitter to producethe RF signal for transmission over the air to an RF receiver. The PAcircuit may have a nonlinear gain characteristic, such as gaincompression occurring at higher power output levels, which can lead tosignal distortion at such higher power levels.

U.S. Pat. No. 6,342,810, for example, mentions a method of compensatingfor amplifier nonlinearities by using predistortion to apply an inversemodel of the amplifier's transfer characteristic to an input signal ofthe amplifier. A goal of such predistortion is to reduce distortion dueto the PA circuit gain nonlinearity.

SUMMARY

The present inventors have recognized, among other things, that allowinga power amplifier (PA) circuit to operate including in its nonlinear(e.g., gain compression) region, such as by using predistortioncompensation, can provide one or more benefits, such as to improveamplifier efficiency and performance, reduce power consumption, reducewaste heat generation, and reduce or avoid the need for active orpassive cooling of the PA circuit, but that using the PA circuit withwideband input signals may present additional challenges that that canincrease distortion and noise in a PA circuit.

A PA circuit operated at least in part in its nonlinear region producesdistortions across a wide frequency band. For example, when a PA circuitis used to amplify a carrier-modulated input signal, the PA circuitgenerates distortion terms centered at the carrier frequency and atharmonics of the carrier frequency. When the input signal has abandwidth that is less than the carrier frequency, predistortioncompensation may be used to correct for distortion terms centered at thecarrier frequency, while distortion terms at the carrier frequencyharmonics are removed by low-pass filtering. When the input signal is awideband or ultrawide band signal with a bandwidth similar to or largerthan the carrier frequency, however, the input signal frequency band mayoverlap with distortion terms at one or more of the carrier frequencyharmonics. This may make it may difficult to use low-pass filtering toremove distortion terms without also degrading the input signal.

Among other things, this document explains how predistortioncompensation can be used to correct distortion terms at the carrierfrequency and to correct distortion terms at one or more carrierfrequency harmonics. For example, the input signal may be and/or beconverted to a complex signal (e.g., a complex baseband signal). Alinear digital predistortion (DPD) circuit may receive the complexbaseband signal and generate a pre-distorted signal linear component. Anonlinear DPD circuit may convert the complex baseband signal to a realsignal and generate a pre-distorted signal nonlinear component from thereal signal.

This overview is intended to provide an overview of subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the presentpatent application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 shows an example of a power amplifier (PA) system with mixed modedigital predistortion (DPD).

FIG. 2 shows one example of the PA circuit of FIG. 1 showing additionaldetails of the mixed mode DPD circuit.

FIG. 3 is a diagram showing bandwidth expansion in an example of thenonlinear DPD circuit.

FIG. 4 is a diagram of an example of the PA circuit of FIG. 1 includingadditional details of the nonlinear DPD circuit.

FIG. 5 is a diagram showing an example of the nonlinear DPD circuitincluding additional details of the complex-to-real circuit.

FIG. 6 is a diagram showing an example of the PA circuit of FIG. 1showing the real DPD circuit implemented with a look-up table (LUT)bank.

FIGS. 7A and 7B are a diagram showing an example of a Finite ImpulseResponse (FIR) filter that may be used to implement the real DPD circuitin addition to or instead of the LUT bank of FIG. 6.

FIGS. 8A, 8B and 8C are a diagram illustrating components of thereal-to-complex circuit and mixer.

FIG. 9 is a diagram showing an example of the PA circuit of FIG. 1 alongwith plots showing example Hilbert filter responses and examples of thepre-distorted nonlinear component ũ.

FIGS. 10A and 10B show a real-mode DPD circuit configured to processlinear terms and a real DPD circuit and real-to-complex circuit.

FIG. 11 is a diagram showing an example of the PA circuit of FIG. 1including additional details of the linear DPD circuit.

FIG. 12 is a diagram showing an example of the PA circuit of FIG. 1including additional details of the training circuit.

FIG. 13 is a diagram showing one example of the PA circuit of FIG. 1including a cable uptilt circuit.

FIG. 14 is a diagram showing an example of a configuration setup of a PAcircuit for training the tilt equalizer.

FIGS. 15A and 15B is a diagram showing one example of the PA circuit ofFIG. 1 including a cable uptilt circuit and another example of atraining circuit.

FIGS. 16 and 17 are a diagram showing the PA circuit of FIG. 1 in theexample configuration of FIG. 13 including modeled power spectraldensities at various positions in the circuit.

FIG. 18 is a diagram showing an example of the PA circuit of FIG. 1configured to consolidate the complex or Hilbert filter and thedecimation filter.

FIG. 19 is a diagram showing an example implementation of the nonlinearDPD circuit including an alternate component configuration.

DETAILED DESCRIPTION

U.S. patent application Ser. No. 15/159,492, entitled “WIDEBAND DIGITALPREDISTORTION,” and filed concurrently herewith is incorporated hereinby reference in its entirety.

FIG. 1 shows an example of a power amplifier (PA) circuit 100 with mixedmode digital predistortion (DPD). The circuit 100 may be utilized indifferent applications including, for example, for RF transmission, suchas cellular network transmissions, and/or for transmissions through acable (e.g., a coaxial cable) of a cable television network or similarnetwork. A mixed mode digital predistortion (DPD) circuit 102 mayreceive as an input signal a complex baseband signal {tilde over (x)}.(Digital Upconverter (DUC) 104, which may be positioned upstream of theDPD circuit 102, is described in more detail below.) The complexbaseband signal {tilde over (x)} may be a complex or analytic signalhaving a real component and a quadrature component. The DPD circuit 102may generate a pre-distorted complex baseband signal {tilde over (v)}.The pre-distorted complex baseband signal {tilde over (v)} may beprovided to a digital-to-analog converter (DAC) 110 (e.g., via acommunications link 108). In some examples, the DAC 110 may alsomodulate the pre-distorted complex baseband signal {tilde over (v)} to acarrier frequency f_(c) to generate a transmission signal v, although inother examples a separate up-converter component may be used. Thetransmission signal v may be provided to a power amplifier 112, such asfor wireless or wired transmission.

A DPD adaptation or training circuit 106 may train the DPD circuit 102,for example, as described herein. An output of the PA 112 may be sampledby an analog-to-digital converter (ADC) 114 to generate a feedbacksignal y. In the example shown in FIG. 1, the ADC 114 also down-convertsthe sampled output of the PA 112 to baseband, although in other examplesa separate down-converter component may be used. The ADC 114 may providethe feedback signal y to the DPD training circuit 106, (e.g., via thecommunications link 116). The DPD training circuit 106 may also receivethe pre-distorted complex baseband signal {tilde over (v)} (and/or areal signal v generated by the DPD circuit 102). Based on thepre-distorted complex baseband signal {tilde over (v)} and the complexbaseband feedback signal {tilde over (y)}, the DPD adaptation circuit106 may generate and/or update the DPD circuit 102. Additional examplesfor training the DPD circuit 102 are described herein.

In the example of FIG. 1, the complex baseband signal {tilde over (x)}is received by the DPD circuit 102 from a digital up-converter (DUC)104. The DUC 104 may be used in various applications, such as cableapplications. For example, the DUC 104 may receive a set of channelsignals. Each channel signal may be a complex signal centered atbaseband. In some cable television applications, each channel signalrepresents one television channel. The DUC 104 may stack the channelsignals to generate the complex baseband signal. In a simple example,the DUC 104 receives four channel signals, each having a bandwidth B,and each centered at a frequency B/2. The first channel signal mayremain centered at B/2. The DUC 104 may translate the second channelsignal to generate a translated second channel signal centered at 3B/2.The DUC 104 may translate the third channel signal to generate atranslated third channel signal centered at 5B/2 and may translate thefourth channel signal to generate a translated fourth channel signalcentered at 7B/2. The DUC 104 may combine the first channel signal, thetranslated second channel signal, the translated third channel signal,and the translated third channel signal to generate the complex basebandsignal. The resulting complex baseband signal may have a bandwidth of4B. Although this example includes four channel signals, more or fewerchannel signals may be acted on by the DUC 104. Also, although thechannel input signals received by the DUC 104 are described as beingreceived at the DUC 104 in complex baseband, in some examples, the DUC104 may receive real channel signals and convert the real channelsignals to the complex baseband signal output. Also, in some examples,the output of the DUC 104 may be expressed as a real signal and may beconverted to complex form by a subsequent circuit component. In someexamples, the DUC 104 is omitted.

The example of FIG. 1 shows a configuration of the circuit 100implemented utilizing a Field Programmable Gate Array (FPGA) 118. Forexample, the DUC 104, DPD circuit 102, and DPD training circuit 106 areshown implemented by the FPGA 118. Components or modules not implementedby the FPGA 118 in FIG. 1 may be implemented using any other suitablehardware. The communications links 108, 116 shown in FIG. 1 may be usedto facilitate communications between the FPGA 118 and the various othercomponents of the circuit 100. The communications links 108, 116 may beconfigured according to any suitable protocol, such as the JESD204serial protocol. Although the communications links 108, 116 are shown onthe FPGA 118, in some examples, communications protocols 108, 116 may beimplemented with different hardware components. Also, in some examples,the DPD adaptation circuit 106 may be implemented off of the FPGA 118,such as at a separate digital signal processor (DSP) (not shown). Forexample, the DPD adaptation circuit 106 may train the DPD circuit 102 ata clock rate slower than the operation of the DPD circuit 102 allowingthe DPD adaptation circuit 106 to be implemented on a slower DSP ratherthan the FPGA 118.

FIG. 2 shows an example of the PA circuit 100 of FIG. 1 showingadditional details of the mixed mode DPD circuit 102. The DPD circuit102 may comprise a linear DPD circuit 202 and a nonlinear DPD circuit204. The linear DPD circuit 202 may operate in complex mode at baseband.The linear DPD circuit 202 may receive the complex baseband signal{tilde over (x)} and provide as output a pre-distorted signal linearcomponent ũ_(l), which may also be a complex signal at baseband. In someexamples, the linear DPD circuit 202 may apply a scalar gain to thecomplex baseband signal {tilde over (x)}. Additional details of thenonlinear DPD circuit 204 are provided herein, for example, withreference to FIG. 11.

In some examples, use of separate linear and nonlinear DPD circuits, asdescribed herein may provide several advantages. For example, real-modeprocessing of the nonlinear correction terms by the nonlinear DPDcircuit 204 may enable the correction of wide bandwidth complex basebandsignals. In some examples, wide bandwidth complex baseband signals mayinclude complex baseband signals having a bandwidth that is about equalto the carrier frequency and/or complex baseband signals where the lowerband edge is less than or equal to about half of the upper band edgeand/or at or close to direct current (DC) (e.g., f=0). For example,real-mode processing at the nonlinear DPD circuit 204 may providecorrection for multiple distortion orders across multiple harmonicfrequency zones of the carrier frequency. On the other hand, performinglinear processing in complex mode may enable the nonlinear DPD circuit204 to utilize a simpler real-to-complex converter, for example, with asimpler filter design, as described herein.

The nonlinear DPD circuit 204 may also receive the complex basebandsignal {tilde over (x)} and provide as output a pre-distorted nonlinearcomponent ũ. The pre-distorted nonlinear component ũ may be a complexsignal at baseband. The pre-distorted signal linear component ũ_(l) andthe pre-distorted nonlinear component ũ may be added at a summer circuit216, resulting in the pre-distorted signal {tilde over (v)}, which mayalso be a complex signal at baseband. In some examples, such as cableimplementations, the pre-distorted signal {tilde over (v)} may bedecimated or otherwise of its sample rate dropped to the same rate asthe complex baseband signal {tilde over (x)}, for example, to facilitatetransmittal via the communication link 108. The nonlinear DPD circuit204 may operate in real mode and, in some examples, at the carrierfrequency f_(c). For example, the nonlinear DPD circuit 204 may comprisecircuits for converting the complex baseband signal {tilde over (x)} toreal mode, modulating the complex baseband signal {tilde over (x)} tothe carrier frequency f_(c) and/or performing other processing. FIG. 2shows various constituent circuits of the nonlinear DPD circuit 204including an interpolator circuit 206, a complex-to-real convertercircuit 208, a real DPD circuit 210, a real-to-complex circuit 212, anda decimator circuit 214.

FIG. 2 also includes plots of examples of the various signals in thecircuit 100. A plot 201 shows one example of the complex baseband signal{tilde over (x)}. In the example of FIG. 2, the complex baseband signalz has a bandwidth of about 1.2 GHz, extending from −0.6 GHz to 0.6 GHz.A plot 203 shows an example of the pre-distorted signal linear componentũ_(l). In some examples, the pre-distorted signal linear component ũ_(l)may have the same bandwidth as the complex baseband signal {tilde over(x)}. For example, the plot 203 shows the pre-distorted signal linearcomponent ũ_(l) with a bandwidth of about 1.2 GHz, extending from −0.6GHz to 0.6 GHz, similar to the plot 201 of the example complex basebandsignal {tilde over (x)}. A plot 205 shows one example of thepre-distorted non-linear component ũ. The pre-distorted nonlinearcomponent ũ may include pre-distorted content 209 that may be in thebandwidth of the complex baseband signal {tilde over (x)}, although someof the pre-distorted content 209 may be out-of-band. The pre-distortednonlinear component ũ may also comprise distortion and/or aliasingcomponents 211. As described herein in more detail with reference toFIG. 9, the distortion content 211 and out-of-band portions of thepre-distorted content 209 may fall below the noise floor of the DAC 110and/or PA 112 and therefore maybe ignored and/or filtered.

FIG. 3 is a diagram showing bandwidth expansion in an example of thenonlinear DPD circuit 204. FIG. 3 shows the complex-to-real circuit 208,the real DPD circuit 210, and the real-to-complex circuit 212 of thenonlinear DPD circuit 204. The complex baseband signal {tilde over (x)}may have an initial bandwidth indicated in FIG. 3 as BW. The initialbandwidth may be any suitable bandwidth. For example, the initialbandwidth may be a wideband or ultrawideband bandwidth within an orderof magnitude of the carrier frequency f_(c). In the example of FIG. 2,the plot 201 of the complex baseband signal {tilde over (x)} has aninitial bandwidth of 1.2 GHz, although complex baseband signals of otherbandwidths may be used.

The various components of the nonlinear DPD circuit 204 may expand andcontract the bandwidth of the processed signals. Converting the complexbaseband signal {tilde over (x)} to real mode at complex-to-real circuit208 may generate a real signal x with double the original bandwidth BW,or 2BW. The real signal x, in some examples, may be translated to thecarrier frequency f_(c) prior to processing by the real DPD circuit 210.For example, converting the complex baseband signal x to real mode mayintroduce negative frequency content that can double the originalbandwidth BW. The real DPD circuit 210 may further expand the bandwidthof the real signal x. For example, the pre-distorted nonlinear componentu may have a bandwidth greater than the bandwidth of the real signal xby a factor equal to the highest order distortion term compensated bythe real DPD circuit 210. In the example of FIG. 3, the highest orderdistortion term to be compensated is of the 3^(rd) order. Accordingly,the real DPD circuit 210 may generate a pre-distorted nonlinearcomponent u with a bandwidth of about three times the bandwidth of thereal signal x, or about 6BW. Conversion of the pre-distorted nonlinearcomponent u to a complex pre-distorted nonlinear component ũ may halvethe bandwidth from 6 BW to 3 BW.

Bandwidth expansion in the nonlinear DPD circuit 204 may increase thedesirable sample frequency f_(s) in the circuit 204. The samplefrequency f_(s) (also sometimes referred to as the sample rate) mayrefer to the number of samples in of the various signals per unit time.Under the Nyquist theorem, the maximum signal frequency that can beaccurately reconstructed from a digital signal is equal to the samplingfrequency f_(s), for complex signals, and equal to ½ of the samplingfrequency (f_(s)/2) for real signals. Accordingly, if the bandwidth ofthe signals processed by the nonlinear DPD circuit 204 is expanded toinclude frequency content at greater than the Nyquist frequency, thenaliasing and/or other distortion may be introduced unless the samplefrequency f_(s) (and hence the Nyquist frequency) is also increased.Referring again to FIG. 2, the interpolator circuit 206 may beconfigured to increase the sampling frequency f_(s) of the complexbaseband signal {tilde over (x)} to allow for bandwidth expansion in thenonlinear DPD circuit 204 without excessive aliasing or distortion. Theinterpolator circuit 206 may increase the sampling frequency to twotimes the highest expected frequency content. In various examples, thehighest expected frequency content in the nonlinear DPD circuit may behighest order distortion term to be corrected. For example, if thehighest order distortion term to be corrected is three (e.g., 3^(rd)order), the interpolator circuit 206 may increase the sampling frequencyby a factor of 2*3, or 6. In some examples, distortion terms with anorder higher than a selected order (e.g., the 3^(rd) order) may drop offsignificantly in magnitude so as to be below the noise floor of the PAcircuit 100. Accordingly, in some examples, the interpolator circuit 206may be configured to increase the sampling frequency of the complexbaseband signal z by a factor of six (6), or other suitable order, evenif higher order distortion terms are to be considered. The interpolatorcircuit 206 and/or decimation circuit 214 may be implemented using anysuitable technology such as, for example, Cascaded Integrator-Comb (CIC)or Hogenauer filtering, half-band filtering, polyphase filtering, etc.

FIG. 4 is a diagram of an example of the PA circuit 100 includingadditional details of the nonlinear DPD circuit 204. In the example ofFIG. 4, the nonlinear DPD circuit 204 may include a real DPD circuit 210that operates at the carrier frequency f_(c). Accordingly, the complexbaseband signal {tilde over (x)} may be translated to the carrierfrequency f_(c) at a mixer 218 to generate a translated complex signal.Up-conversion may be accomplished, for example, by multiplying thecomplex baseband signal {tilde over (x)} by a complex representation ofthe carrier frequency, given by [1]:e ^(jω) ^(c) ^(t)  [1]In [1], ω_(c) may be the angular representation of the carrier frequencyf_(c). For example, ω_(c) may be equal to 2πf_(c). Before being summedwith the pre-distorted signal linear component ũ_(l), the pre-distortednonlinear component ũ may be translated back to baseband. For example,the pre-distorted nonlinear component ũ at the carrier frequency f_(c)may be multiplied by a complex inverse representation of the carrierfrequency, given by [2]:e ^(−jω) ^(c) ^(t)  [2]In FIG. 4, translation to the carrier frequency occurs after theinterpolator circuit 206 and prior to the decimator circuit 214. In someexamples, however, translating may occur prior to the interpolatorcircuit 206 and/or translating may occur after the decimator circuit214.

FIG. 5 is a diagram showing an example of the nonlinear DPD circuit 204including additional details of the complex-to-real circuit 208. In theexample of FIG. 5, the complex-to-real circuit 208 and the mixer 218 arecombined into a complex baseband-to-real circuit 212. FIG. 5 includes aplot 222 showing one example of the complex baseband signal {tilde over(x)}. The example plot 222, similar to the example plot 201, has abandwidth of 1.2 GHz centered at DC. Mixer 218 may translate the complexbaseband signal {acute over (x)} to the carrier frequency, resulting ina 1.2 GHz bandwidth signal centered at f_(c). The complex-to-realcircuit 208 may generate the real signal x, which may have a 1.2 GHzfrequency component centered at f_(c) and a 1.2 GHz frequency componentcentered at −f_(c) as shown by plot 224.

FIG. 6 is a diagram showing an example of the PA circuit 100 showing thereal DPD circuit 210 implemented with a look-up table (LUT) bank 250.The real DPD circuit 210, as described herein, may receive a real signalx(n) that is a real component of the complex baseband signal ztranslated to the carrier frequency f_(c), for example, represented by[3]:x(n)=

e{{tilde over (x)}(n)e ^(jω) ^(c) ^(t)}  [3]The LUT bank 250 may comprise M LUTs 252 a, 252 b, 252 i, where M is thememory depth of the LUT bank 250. For example, the LUT bank 250 maydetermine a value x(n) at a first sample n. The value may be used as anindex to select a value from a first LUT 252 a. The value for x(n) maybe provided to a cascade of delay circuits 254 b, 254 i, which maygenerate delayed values of x that may be provided to LUTs 252 b, 252 iat different memory levels. For example, the output of LUT 252 b may bebased on x(n−1) and the output of LUT 252 i may be based on x(n−M). Thevalues for each of the MA+1 LUTs 252 a, 252 b, 252 i may be summed atsummer 256 to yield the pre-distorted nonlinear component u. Examplevalues for the LUTs as a function of x(n) are shown by a plot 258.

A general expression for a DPD pre-distorted signal may be given byEquation [4] below:v(n)=Σ_(m=0) ^(M)Σ_(k=0) ^(K) h _(k)(m)x(n−m)^(k)  [4]Equation [4] represents a memory polynomial approximation of a generalVolterra series. In some examples, the Volterra series itself, or othersuitable approximations may be used. In Equation [4], v is thepre-distorted signal, M is the memory depth, h_(k)(m) is a coefficientof the memory polynomial or other model of the distortion caused by thePA 112. The coefficient h_(k)(m) may be a function of k, which is theorder of the term corrected, and m, which is the memory depth. M may bethe highest memory depth of the correction, and K may be the highestorder distortion term to be corrected. From Equation [4], a onedimensional look-up function

_(mK) may be derived, as indicated by Equation [5] below:

m ⁢ ⁢ K ⁢ { x ⁡ ( n - m ) } = ∑ k = 0 K ⁢ ⁢ h k ⁡ ( m ) ⁢ x ⁡ ( n - m ) k [ 5 ]Accordingly, each look up table (LUT) value

_(mK) at a given memory depth m may be based on a sum over all correcteddistortion orders of the product of order-specific coefficients and thereal component value at the memory depth. A pre-distorted signal v(n)may be found by summing the outputs of the M+1 LUTs, as given byEquation [6] below:

v ⁡ ( n ) = ∑ m = 0 M ⁢ ⁢ mK ⁢ { x ⁡ ( n - m ) } [ 6 ]For a mixed-mode real DPD circuit, such as 210, the linear memorylessterm of v(n) may be omitted to generate the pre-distorted nonlinearcomponent u(n), which may be given by [7] below:u(n)=v(n)−h ₁(0)x(n)  [7]To remove the linear memoryless term from the LUT bank 250, in someexamples, the output of the memoryless LUT 252 a may be modified fromthe general form given by Equation [5] above to the form given byEquation [8] below:

0 ⁢ ⁢ K ⁢ { x ⁡ ( n ) } = h 0 + ∑ k = 2 K ⁢ ⁢ h k ⁡ ( m ) ⁢ x ⁡ ( n ) k [ 8 ]Accordingly, values from the memoryless LUT 252 a may omit first orderterms from the indicated sum. Correction for first order terms may beaffected, for example, by the linear DPD circuit 202. Additional LUTS252 b, 252 i may be configured, for example, as set forth about inEquation [5].

FIGS. 7A and 7B are a diagram showing one example of a Finite ImpulseResponse (FIR) filter 260 that may be used to implement the real DPDcircuit 210 of the nonlinear DPD circuit 204 in addition to or insteadof the LUT bank 250. The FIR filter 260 may comprise taps 264 b, 264 i.The number of taps 264 b, 264 i may depend on the memory depth M of thereal DPD circuit 210. Each tap 264 b, 264 i may receive an output of acorresponding power circuit 252 b, 252 i that may raise a current valueof the real signal x to a power corresponding to the order of the tap264 b, 264 i. As shown, a first order tap (e.g., a tap with coefficienth₁(m)) is omitted to generate the pre-distorted nonlinear component u.Example values for the taps 264 b, 264 i as a function of memory depth mare shown by plot 268. In the example shown in FIGS. 7A and 7B, no firstorder or linear FIR filter is included in the real DPD circuit 210. Insome examples, a linear FIR may be included, but the linear FIR may omitits zero memory tap.

FIGS. 8A, 8B and 8C are a diagram illustrating components of thereal-to-complex circuit 212 and mixer 220. The output of the real DPDcircuit 210 may be a pre-distorted nonlinear component u that is in realmode and centered at the carrier frequency. This is indicated by plot280, which shows the pre-distorted nonlinear component u. For example,the plot 280 shows positive and negative frequency content centered onthe carrier frequency fc and at the negative carrier frequency −f_(c).The example of the real-to-complex circuit 212 shown in FIGS. 8A, 8B and8C includes a Hilbert transform FIR filter 286 and a delay circuit 288.The real signal u may be the real component of the complex output of thecircuit 212. The FIR filter 286 may yield a quadrature component of thecomplex output of the circuit 212. The real and quadrature componentsmay be summed at a summer 290 to generate the complex output of thecircuit 212. Plot 282 shows an example of the complex output of thecircuit 212. This output may be translated to baseband, which may yieldthe pre-distorted nonlinear component ũ, centered at baseband, indicatedby example plot 284.

As shown, the primary frequency content of complex output of the circuit212 is centered on the carrier frequency f_(c), with content from thereal signal that was at the negative carrier frequency f_(c) suppressedby an image reduction ratio (IRR). The Hilbert filter 286 may beselected to achieve an IRR that brings the negative frequency content ofthe complex output of the circuit 212 to an acceptably low level. TheIRR achieved by the Hilbert filter 286 may depend on the number of tapsof the Hilbert filter 286. Plot 284 shows tap values for a thirty-three(33) tap Hilbert filter 286. Plot 285 shows an example magnituderesponse 287 and phase response 289 of a 33 tap Hilbert filter centeredat baseband, although the output of the filter 286 may be centered atthe carrier frequency f_(c).

FIG. 9 is a diagram showing an example of the PA circuit 100 along withplots showing example Hilbert filter responses and examples of thepre-distorted nonlinear component ũ. For example, plot 302 showsmagnitude responses in the negative frequency range for Hilbert filterswith different numbers of taps. For example, a 32 tap Hilbert filter, asshown, may provide an image rejection ratio (IRR) of about 25 dB. A 64tap filter may provide an IRR of about 55 dB. A 96 tap filter mayprovide an IRR of about 75 dB. A 128 tap filter may provide an IRR ofabout 128 dB. Plot 304 shows the pre-distorted nonlinear component ũ.The pre-distorted nonlinear component includes predistortion terms 306as well as noise 308. Noise 308 may result from images generated by theHilbert filter of the complex-to-real circuit 212, aliases fromup-sampling and/or down-sampling, etc. The plot 304 also shows thepre-distorted signal linear component ũ_(l) in phantom.

In some examples, the noise floor of the PA circuit 100 may be about 54dB below the magnitude of the pre-distorted signal {tilde over (v)}. Asshown by the plot 304, this is also about 54 dB below the magnitude ofthe pre-distorted signal linear component ũ_(l). Also shown by the plot304, however, is that the predistortion terms 306 of the pre-distortednonlinear component ũ are already about 30 dB to the carrier frequency(dBc) below the magnitude of the pre-distorted signal linear componentũ_(l). Accordingly, the Hilbert filter 286 of the real-to-complexcircuit 212 may achieve an effective IRR of −54 dB by adding anadditional ˜25 dB to the 30 dB reduction already built in to thepre-distorted nonlinear component. This may enable the of a Hilbertfilter with fewer taps (e.g., 32 or 33) instead of a larger, moreexpensive filter. Accordingly, in some examples, splitting the DPDcircuit 102 between the linear DPD circuit 202 and the nonlinear DPDcircuit 204, including the real DPD circuit 210, may allow the DPDcircuit 102 to enjoy the advantages of real-mode processing whileminimizing the disadvantages. For example, the mixed-mode DPD circuit102 may enjoy simplified correction of multiple orders of distortion atmultiple harmonic frequency zones in the real DPD circuit 210. Separateprocessing of the linear term, however, may reduce the complexity of thereal-to-complex converter, as described above.

This is further illustrated by FIGS. 10A and 10B, which show a real-modeDPD circuit 320 configured to process linear terms and the real DPDcircuit 210 and real-to-complex circuit 212 described herein. Plot 325shows an example frequency response of the output of the real-mode DPD322, indicated as the real input or i/p of the real-to-complex circuit324. The real input includes two negative frequency peaks at 200 MHz and800 MHz as well as corresponding negative frequency peaks at −200 MHzand −800 MHz. The complex output (e.g., the output of thereal-to-complex circuit 324 utilizing a 32 tap Hilbert filter) is alsoshown. As illustrated, the output comprises negative frequency peaks at−200 MHz and −800 MHz that are reduced by about 40 dB from thecorresponding negative frequency peaks of the input. In applicationswith a noise floor more than about 40 dB below the signal peak, then,the 32 tap Hilbert filter may be insufficient. Plot 328 also shows anexample input and output to the real-to-complex circuit 212. Asdescribed herein, the input to the real-to-complex circuit 212 omitslinear terms. Accordingly, the −200 MHz and −800 MHz frequency peaks ofthe input are already about 20 dB lower in plot 328 than in plot 325.Accordingly, the 32 tap Hilbert filter provides a total IRR of about 60dB, which may be acceptable for some applications.

FIG. 11 is a diagram showing one example of the PA circuit 100 includingadditional details of the linear DPD circuit 202. In some examples, thelinear DPD circuit 202 may include a single scalar gain stage h₁(0). Insome examples, the single scalar gain stage h₁(0) may be equivalent tothe linear tap or term omitted from the real DPD circuit 210, asdescribed herein. The value of the scalar gain h₁(0) may be set by thetraining circuit 106, for example, as described herein. In someexamples, a complex time align circuit 203 may be included upstream ofthe linear DPD circuit 202 prior to the summer 216. The complex timealign circuit 203 may be configured to apply a delay m, to the output ofthe linear DPD circuit 202. The delay τ_(D) may be selected to align thetime and phase of the pre-distorted signal linear component ũ_(l) withthe pre-distorted nonlinear component u. For example, the complex timealign circuit 203 may compensate for propagation delays and/or phasechanges to the pre-distorted nonlinear component ũ due to additionalprocessing (e.g., from the interpolator circuit 206, the real-to-complexcircuit 208, the real DPD circuit 210, the complex-to-real circuit 212,the decimator circuit 214, the mixers 218, 220, etc.).

FIG. 12 is a diagram showing one example of the PA circuit 100 includingadditional details of the training circuit 106. The example trainingcircuit 106 shown in FIG. 12 operates in real-mode by comparing a realpre-distorted signal v to the feedback signal y. For example, thepre-distorted complex baseband signal {tilde over (v)} provided to theDAC 110 and PA 112 may be a complex signal at baseband. Thepre-distorted nonlinear component u may be real, but may also excludethe linear, memoryless component, which is determined by the linear DPDcircuit 202. Accordingly, the training circuit 106 may receive the inputsignal x at a combined real DPD circuit 322. The combined real DPDcircuit 322 may find the combined real pre-distorted signal v. Thecombined real DPD circuit 322 may implement a complete real DPD. Forexample, the combined real DPD circuit 322 may utilize a bank of LUTsand/or an FIR filter that includes linear memoryless terms. For example,the real pre-distorted signal v may be determined from the input signalx as described by Equation [4] above. A reference model filter circuit324 and time alignment circuit 326 may be applied to the realpre-distorted signal v to emulate the PA 112 and align the referenceoutput z with the feedback signal y.

The feedback signal y may be processed by a rate match circuit 332 toincrease its sampling frequency to match the sampling frequency of theinput signal x and the real pre-distorted signal v. The rate-matchedfeedback signal may be provided to a nonlinear inverse model circuit330. The nonlinear inverse model circuit 330 may generate an actualoutput {circumflex over (z)}. A difference circuit 334 may find adifference E between the actual output {circumflex over (z)} and thereference output z. The difference c may be provided to an adaptioncircuit that, in conjunction with the nonlinear inverse model circuit330, may generate a coefficient matrix h. A linear, memoryless termh₁(0) of the matrix h may be provided to the linear DPD circuit 202. Theother coefficients of the coefficient matrix h may be provided to thereal DPD circuit 210 to be used as FIR filter coefficients and/or to aLUT construction circuit 320, which may utilize the coefficient matrix hto generate LUTs 252 a, 252 b, 252 i, for example, as described abovewith respect to Equation [8].

The mixed mode DPD circuit 102 described herein may be used for RFtransmission and/or higher bandwidth cable implementations. In someexamples where the mixed mode DPD circuit 102 is used for cableimplementations, a cable uptilt circuit 350 positioned prior to the PA112 may apply an “uptilt” frequency modification to the pre-distortedsignal v. The uptilt frequency modification may compensate for frequencydependent signal loss exhibited by some cables. For example, a cable mayexhibit a high frequency rolloff characteristic of about 2 dB of signalamplitude reduction per 100 MHz of frequency, such as at frequenciesabove 50 MHz. The uptilt frequency modification may amplify higherfrequency portions of the signal that are attenuated by the cable so asto reduce frequency-dependent distortions at the signal destination.

FIG. 13 is a diagram showing an example of the PA circuit 100 includinga cable uptilt circuit 350. The cable uptilt circuit 350 may be ananalog circuit positioned prior to the PA 112 and configured with ahighpass frequency gain expansion characteristic, for example tocompensate for high frequency rolloff characteristic of a cable or othertransmission medium downstream of the PA 112. For example, the cableuptilt circuit 350 may have a highpass frequency gain expansioncharacteristic of about +2 dB/100 MHz, for example, shown by plot 360.

In the example of FIG. 13, the nonlinear DPD circuit 204 may alsoinclude a tilt reference filter circuit 352 positioned prior to the realDPD circuit 210 and a tilt equalizer circuit 354 positioned after thereal DPD circuit 210. For example, absent the tilt reference filtercircuit 352 and tilt equalizer circuit 354, the DPD circuit 102 may tendto counteract the cable uptilt circuit 350. The tilt reference filtercircuit 352 may include a frequency response that can be specified andset to match a desired uptilt observed at the output of the PA circuit100. Like the analog uptilt circuit 350, the digital uptilt filtercircuit 352 can provide a highpass frequency gain expansioncharacteristic (e.g., +2 dB/100 MHz amplification for frequenciesexceeding 50 MHz) of a cable to offset or compensate for the lowpassfrequency dependent signal loss of the cable 102. This is illustrated bythe plot 356, which shows a +2 dB/100 MHz gain characteristic forfrequencies above 50 MHz.

The tilt equalizer circuit 354 may be may include a digital equalizerhaving a frequency modification setting that has a lowpass filtercharacteristic, such as shown in plot 258, such as approximately −2.2dB/100 MHz for frequencies above 50 MHz, which may be approximately theinverse of the frequency modification provided by the digital tiltreference filter circuit 352 and analog uptilt circuit 350. Theparticular frequency modification to be provided by the equalizercircuit 354 can be selected to compensate for or eliminate the effect ofthe analog uptilt circuit 350 interposing a frequency dependent uptiltgain and group delay variation between the real DPD circuit 210 and thePA 112. The particular frequency modification provided by the tiltequalizer circuit 354 may be selected to provide a substantiallyconstant gain versus frequency, a substantially constant group delayversus frequency or both, such as at the output of the tilt equalizercircuit 354. In some examples, the tilt equalizer circuit may also beconfigured to correct for other frequency dependent artifacts resulting,for example, from low pass filtering, decimation, and Hilbert transform.The tilt reference filter circuit 352 and tilt equalizer circuit 354 maybe run in real mode at the up-sampled sampling frequency of thenonlinear DPD circuit 204. In some examples, a mixed mode DPD circuit102 having separate linear and nonlinear DPD circuits 202, 204 asdescribed herein, may omit the tilt equalizer circuit 354. For example,the output of the real DPD circuit 210 may be provided to thereal-to-complex circuit 212, mixer 220 and/or decimator circuit 214.

FIG. 14 is a diagram showing one example of a configuration setup 370 ofa PA circuit for training the tilt equalizer 354. A broadband stimuluscircuit 372 may generate a broadband stimulus signal indicated in FIG.14 by {tilde over (x)}. The broadband stimulus signal may be provided tothe interpolator circuit 206, mixer 218, and complex-to-real convertercircuit 208, for example, as described herein, to generate a real signalx. The tilt equalizer 354 may receive the real signal x. An output ofthe tilt equalizer 354 may be provided to the real-to-complex converter212, the mixer 220, and the decimator circuit 212, as described herein,to generate an output signal. The output signal may be provided to theDAC 110, analog cable uptilt filter 350 and PA 112.

An uptilt training circuit 374 may generate a tap coefficient vector θof tap coefficients θ_(i) for the uptilt equalizer circuit 354. Forexample, an output of the uptilt equalizer circuit 354, indicated inFIG. 14 by x, may be used to generate a reference output z. A timealignment circuit 376 and reference model circuit 378 may act on theuptilt equalizer output x to generate the reference output z. Similarly,the feedback signal y may be rate matched to the up-sampled samplingfrequency of the interpolator circuit 206 by rate match circuit 332.Linear inverse model circuit 382 may receive the feedback signal y andgenerate an actual output {circumflex over (z)}. A difference circuit380 may receive the reference output z and actual output {circumflexover (z)} and, in conjunction with the linear inverse model circuit 382,generate the tap coefficient vector θ.

FIGS. 15A and 15B are a diagram showing one example of the PA circuit100 of FIG. 1 including a cable uptilt circuit 350 and another exampleof a training circuit 106. The training circuit 106′ is another exampleof a training circuit that may be used, in some examples, in place ofthe training circuit 106 shown in FIG. 12. The nonlinear DPD circuit 204shown in FIGS. 15A and 15B may be similar to the nonlinear DPD circuit204 as configured in FIG. 13 with the tilt reference filter 352 and tiltequalizer 354 positioned and the cable uptilt circuit 350 as shown.

The configuration of the training circuit 106 shown in FIGS. 15A and 15Breceives an output of the power amplifier 112, for example, via ADC 114.In some examples, a communication link, such as 116 in FIG. 12, may alsobe included. The training circuit 106 may also receives an output of thereal DPD circuit 210. A time alignment circuit 502 and rate matchcircuit 504 may match the timing and the sampling frequencies of theoutput of the real DPD circuit 210 and the output of the power amplifier112. For example, propagation delays from the components of thenonlinear DPD circuit, the power amplifier 112 and other variouscomponents may cause the output of the power amplifier 112 to be delayedrelative to the output of the real DPD circuit 210. Also, as describedherein, the sampling frequency after the interpolator 206 and before thedecimator 214 (e.g., where the output of the real DPD circuit 210 istaken) may be higher than the sampling frequency at the ADC 114. Forexample, the sampling frequency at the ADC may match the samplingfrequency of the complex baseband signal.

Values for the output of the real DPD circuit 210 and the poweramplifier 112 may be stored at respective memory buffers 506, 508. Forexample, memory buffer 506, which receives the output of the real DPDcircuit 210, may provide to a basis vector generation circuit 510 a DPDvector constructed from values of the output of the real DPD circuit 210stored at the memory buffer 506 over time. The basis vector generationcircuit 510 may shift the DPD vector by the carrier frequency 512 togenerate a basis vector matrix, represented in FIGS. 15A and 15B by Ψ. Alinear solver circuit 516 may receive the basis vector matrix Ψ and acorresponding observed vector y matrix generated from the memory buffer508. The linear solver circuit 516 may generate and/or solve a system oflinear equations equivalent to Ψ*h=y, where h is the coefficient matrixfor the DPD circuit 100. A linear, memoryless term h₁(0) of thecoefficient matrix h may be provided to the linear DPD circuit 202. Theother coefficients of the coefficient matrix h may be provided to thereal DPD circuit 210 to be used as FIR filter coefficients and/or to aLUT construction circuit 518, which may utilize the coefficient matrix Ato generate LUTs 252 a, 252 b, 252 i, for example, as described abovewith respect to Equation [8].

In the example of FIG. 16, the training circuit the basis vector matrixΨ is generated from an output of the real DPD circuit 210. In someexamples, the example configuration shown in FIGS. 15A and 15B couldalso be configured to generate the basis vector matrix Ψ from an inputof the real DPD circuit 210, for example, similar to the example of FIG.12.

In the examples of FIGS. 13 and 15, the tilt reference filter 352 andtilt equalizer 354 are implemented in the nonlinear DPD circuit 204, forexample, at the higher sampling frequency fs, and therefore higherclocking frequency, that is implemented after the interpolator circuit206 and before the decimator circuit 214. In some examples, however, thetilt reference filter 352 and/or tilt equalizer 354 may be positioned atany suitable position in the nonlinear DPD circuit 204. For example, thetilt reference filter 352 may be positioned prior to the interpolatorcircuit 206 and/or the tilt equalizer circuit 354 may be positionedafter the decimator circuit 214. This may allow the circuits 352, 354 tooperate in complex mode and, therefore, at a lower sampling frequencyand clock rate.

FIGS. 16-17 are a diagram showing the PA circuit 100 in the exampleconfiguration of FIG. 13 including modeled power spectral densities atvarious positions in the circuit 100. A plot 401 shows the powerspectral density of the example complex baseband signal {tilde over(x)}. The example complex baseband signal {tilde over (x)} comprises twofrequency components, one centered at about 300 MHz and another centeredat about −300 MHz. A plot 403 shows an input to the real DPD circuit210. The input shown at plot 403 may be an up-sampled real-mode signalwith frequency content centered at 200 MHz, 800 MHz, −200 MHz, and −800MHz. The input shown at the plot 403 may have been processed by theinterpolator circuit 206 the mixer 218, the complex-to-real circuit 208,and the tilt reference filter 352. For example, the bandwidth of theinput signal shown in plot 403 is between about −900 MHz and 900 MHz, orabout 1.8 GHz, which is more than double the bandwidth of the complexbaseband signal {tilde over (x)}. Also, the input signal shown in plot403 is a real signal, with instances of the frequency components of thecomplex baseband signal {tilde over (x)} both in the real and complexfrequency domains. The input of the plot 403 also demonstrates tiltfiltering. For example, the peaks of the frequency components in thepositive frequency domain are tilted up to the right.

A plot 407 shows an example pre-distorted nonlinear component u prior totilt equalizing and complex conversion. A plot 408 shows an example ofthe pre-distorted nonlinear component u after tilt equalizing. A plot410 shows the complex pre-distorted nonlinear component ũ. A plot 412shows the pre-distorted signal linear component ũ_(l). Plot 414 showsthe pre-distorted complex baseband signal {tilde over (v)}. Plot 416shows the pre-distorted transmission signal v after the analog uptiltcircuit 350.

FIG. 18 is a diagram showing one example of the PA circuit 100configured to consolidate the complex or Hilbert filter and thedecimation filter. For example, as described herein with respect toFIGS. 8-10, the real-to-complex converter circuit 212 includes a Hilbertor similar FIR filter 286 for determining the quadrature component ofthe complex pre-distorted nonlinear component ũ. FIG. 18 shows apassband 402 of the complex or Hilbert filter 286 of the real-to-complexconverter circuit 212. As illustrated, the passband 402 is between zeroand the half the sampling frequency Fs. Similarly, decimator circuit 214may comprise a low-pass decimation filter for anti-aliasing filter. Alsoshown is a passband 404 of an example decimation filter. The passband404 is centered at zero or direct current and may have a bandwidth aboutequal to the bandwidth of the complex baseband signal {tilde over (x)}.The decimator circuit 214 may include the decimation filter shown bypassband 404 and a decimator 406.

FIG. 18 also shows an alternate configuration 400 of the real-to-complexcircuit 212 and decimation circuit 214. For example, in the alternateconfiguration 400, the real-to-complex filter comprises a Hilbert orcomplex decimation filter with a passband 409. The passband 409 may be acombination of the passband 402 and the passband 404. For example, thepassband 409 may have a bandwidth about equal to the bandwidth of thecomplex baseband signal {tilde over (x)}. Because the complex decimationfilter is positioned prior to the mixer 220, its passband may becentered at the carrier frequency f_(c). After application of thecomplex decimation filter, the decimator 406 may be applied. Forexample, in the configuration 400, the decimation circuit 214 mayinclude the decimator 406 but may omit a decimation filter.

FIG. 19 is a diagram showing an example implementation of the nonlinearDPD circuit 204 including an alternate component configuration. In theexample of FIG. 19, a four times (4×) multirate factor is used.Interpolation is performed in two stages, a first stage interpolatorcircuit 602 and a second stage interpolator circuit 608. The first stageinterpolator circuit 602 includes two one-half (½) band finite impulseresponse (FIR) filters 622A, 622B. For example, the FIR 622A may receivethe real portion of the complex baseband signal {tilde over (x)} whilethe FIR 622B may receive the imaginary or quadrature portion of thecomplex baseband signal {tilde over (x)}. The second stage interpolatorcircuit 608 may comprise a single FIR. For example, the second stageinterpolator circuit 608 may be positioned after the complex basebandsignal is converted to a real signal. Each interpolator stage circuit602, 608 may increase the sampling frequency or bandwidth of theincoming signal by a factor of two (2×), resulting in a total bandwidthincrease to four times (4×) the bandwidth of the complex baseband signal{tilde over (x)}.

Interposed between the first and second stage interpolator circuits 608are a mixer circuit 604 and an optional tilt reference circuit 606. Themixer circuit 604 may multiply the respective outputs of the FIRs 622A,622B by the carrier frequency and sum the results. The output of themixer circuit 604 may be a real equivalent of the complex basebandsignal, shifted by the carrier frequency. The optional tilt referencefilter circuit 606 and optional tilt equalizer circuit 612 may operate,for example, as described herein with respect to the circuits 352, 354described herein. The real DPD circuit 610 may function in a mannersimilar to the real DPD circuit 210 described herein.

In the example of FIG. 19, decimation, like interpolation, may beperformed by a first stage decimation circuit 614 and a second stagedecimation circuit 620. The first stage decimation circuit 614 mayoperate on the real output of the real DPD circuit 610 (and optionaltilt equalizer circuit 612) and may, therefore, utilize a single FIR.First stage decimation circuit may operate to reduce the bandwidth ofthe output of the real DPD circuit by a factor of two, or down to twotimes (2×) the bandwidth of the complex baseband signal {tilde over(x)}. Interposed between the decimation circuits 614, 620 is a Hilbertfilter circuit 616 to convert the signal to complex mode and a complexbaseband circuit to shift the signal back to baseband. Accordingly, thesecond stage decimation circuit 620 may comprise first and second FIRs624A, 624B for the respective real (624A) and complex or quadrature(624B) components of the output signal.

VARIOUS NOTES & EXAMPLES

Example 1 is a system comprising: a linear digital predistortion (DPD)circuit programmed to generate a pre-distorted signal linear componentbased at least in part on a complex baseband signal; a nonlinear DPDcircuit programmed to generate a pre-distorted signal nonlinearcomponent based at least in part on the complex baseband signal; and amixer circuit programmed to generate a pre-distorted signal based atleast in part on the pre-distorted signal linear component and thepre-distorted signal nonlinear component.

In Example 2, the subject matter of Example 1 optionally includes acomplex-to-real circuit programmed to determine a real component basedat least in part on the complex baseband signal.

In Example 3, the subject matter of any one or more of Examples 1-2optionally include an interpolator circuit to up-sample the complexbaseband signal to generate an up-sampled complex baseband signal; andan up-converter circuit to translate the up-sampled complex basebandsignal to a carrier frequency to generate an up-converted complexsignal, wherein the mixer circuit is also to generate the pre-distortedsignal based at least in part on the up-converted complex signal.

In Example 4, the subject matter of any one or more of Examples 1-3optionally include wherein the nonlinear DPD circuit is also programmedto: determine a real component based at least in part on the complexbaseband signal; determine a first value for the real component at afirst time; and select from a first nonlinear look up table (LUT) afirst LUT value based at least in part on the first value for the realcomponent, wherein the first LUT value is a based at least in part on asum of terms omitting a first order term.

In Example 5, the subject matter of Example 4 optionally includeswherein the nonlinear DPD circuit is also programmed to: select from asecond LUT a second LUT value based at least in part on a second valuefor the real component at a second time before the first time; anddetermine a sum of the first LUT value and the second LUT value, whereinthe pre-distorted signal nonlinear component is based at least in parton the sum.

In Example 6, the subject matter of any one or more of Examples 1-5optionally include wherein the nonlinear DPD circuit is also programmedto: determine a real component based at least in part on the complexbaseband signal; determine a first value for the real component at afirst time; determine an Ith order power of the first value; apply anIth order tap coefficient to the Ith order power of the first value togenerate an Ith order tap value; determine a Jth order power of thefirst value; and apply a Jth order tap coefficient to the Jth orderpower of the first value to generate a Jth order tap value, wherein thepre-distorted signal nonlinear component is based at least in part a sumof the Ith order tap value and the Jth order tap value.

In Example 7, the subject matter of any one or more of Examples 1-6optionally include wherein the nonlinear DPD circuit is furtherprogrammed to: determine a real-mode pre-distorted nonlinear component;and convert the real-mode pre-distorted nonlinear component to a complexpre-distorted nonlinear component.

In Example 8, the subject matter of Example 7 optionally includeswherein the nonlinear DPD circuit is further programmed to translate thecomplex pre-distorted nonlinear component to baseband.

In Example 9, the subject matter of any one or more of Examples 1-8optionally include wherein the linear DPD circuit is further programmedto apply a scalar gain stage to the complex baseband signal.

In Example 10, the subject matter of any one or more of Examples 1-9optionally include a complex time align circuit programmed to align thepre-distorted linear component and the pre-distorted nonlinearcomponent.

In Example 11, the subject matter of any one or more of Examples 1-10optionally include a tilt reference filter circuit to apply a highpassfrequency gain characteristic to a real component of the complexbaseband signal to generate a tilt reference real component, wherein thenonlinear DPD circuit is also programmed to determine a tilt referencereal pre-distorted signal nonlinear component based at least in part onthe tilt reference real component.

In Example 12, the subject matter of any one or more of Examples 1-11optionally include wherein the nonlinear DPD circuit is furtherprogrammed to generate a real pre-distorted signal nonlinear componentat a carrier frequency, further comprising: a complex decimation filtercircuit programmed to apply a passband to the real pre-distorted signalnonlinear component to generate a quadrature pre-distorted signalnonlinear component, wherein the passband has a bandwidth matching abandwidth of the complex baseband signal centered at the carrierfrequency; and a demodulation circuit programmed to demodulate thequadrature pre-distorted signal nonlinear component to baseband.

Example 13 is a method comprising: receiving a complex baseband signal;generating a pre-distorted signal linear component based at least inpart on the complex baseband signal; generating a pre-distorted signalnonlinear component based at least in part on the complex basebandsignal; and generating a pre-distorted signal based at least in part onthe pre-distorted signal linear component and the pre-distorted signalnonlinear component.

In Example 14, the subject matter of Example 13 optionally includesdetermining a real component based at least in part on the complexbaseband signal.

In Example 15, the subject matter of any one or more of Examples 13-14optionally include up-sampling the complex baseband signal to generatean up-sampled complex baseband signal; and translating the up-sampledcomplex baseband signal to a carrier frequency to generate anup-converted complex signal, wherein generating the pre-distorted signalis also based at least in part on the up-converted complex signal.

In Example 16, the subject matter of any one or more of Examples 13-15optionally include determining a real component based at least in parton the complex baseband signal; determining a first value for the realcomponent at a first time; and selecting from a first nonlinear look uptable (LUT) a first LUT value based at least in part on the first valuefor the real component, wherein the first LUT value is a based at leastin part on a sum of terms omitting a first order term.

In Example 17, the subject matter of Example 16 optionally includesselecting from a second LUT a second LUT value based at least in part ona second value for the real component at a second time before the firsttime, and determining a sum of the first LUT value and the second LUTvalue, wherein the pre-distorted signal nonlinear component is based atleast in part on the sum.

In Example 18, the subject matter of any one or more of Examples 13-17optionally include determining a real component based at least in parton the complex baseband signal; determining a first value for the realcomponent at a first time; determining an Ith order power of the firstvalue; applying an Ith order tap coefficient to the Ith order power ofthe first value to generate an Ith order tap value; determining a Jthorder power of the first value; and applying a Jth order tap coefficientto the Jth order power of the first value to generate a Jth order tapvalue, wherein the pre-distorted signal nonlinear component is based atleast in part a sum of the Ith order tap value and the Jth order tapvalue.

In Example 19, the subject matter of any one or more of Examples 13-18optionally include determining a real-mode pre-distorted nonlinearcomponent; and converting the real-mode pre-distorted nonlinearcomponent to a complex pre-distorted nonlinear component.

In Example 20, the subject matter of any one or more of Examples 13-19optionally include applying a scalar gain stage to the complex basebandsignal.

In Example 21, the subject matter of any one or more of Examples 13-20optionally include a complex time align circuit programmed to align thepre-distorted linear component and the pre-distorted nonlinearcomponent.

In Example 22, the subject matter of any one or more of Examples 13-21optionally include applying a highpass frequency gain characteristic toa real component of the complex baseband signal to generate a tiltreference real component; determining a tilt reference realpre-distorted signal nonlinear component based at least in part on thetilt reference real component; and applying a tilt equalizer to the tiltreference real pre-distorted signal to generate the real pre-distortedsignal nonlinear component.

In Example 23, the subject matter of any one or more of Examples 13-22optionally include generating a real pre-distorted signal nonlinearcomponent at a carrier frequency; applying a passband to the realpre-distorted signal nonlinear component to generate a quadraturepre-distorted signal nonlinear component, wherein the passband has abandwidth matching a bandwidth of the complex baseband signal centeredat the carrier frequency; and demodulating the quadrature pre-distortedsignal nonlinear component to baseband.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Geometric terms, such as “parallel”, “perpendicular”, “round”, or“square”, are not intended to require absolute mathematical precision,unless the context indicates otherwise. Instead, such geometric termsallow for variations due to manufacturing or equivalent functions. Forexample, if an element is described as “round” or “generally round,” acomponent that is not precisely circular (e.g., one that is slightlyoblong or is a many-sided polygon) is still encompassed by thisdescription.

The term “circuit” can include a dedicated hardware circuit, ageneral-purpose microprocessor, digital signal processor, or otherprocessor circuit, and may be structurally configured from a generalpurpose circuit to a specialized circuit such as using firmware orsoftware.

Any one or more of the techniques (e.g., methodologies) discussed hereinmay be performed on a machine. In various embodiments, the machine mayoperate as a standalone device or may be connected (e.g., networked) toother machines. In a networked deployment, the machine may operate inthe capacity of a server machine, a client machine, or both inserver-client network environments. In an example, the machine may actas a peer machine in peer-to-peer (P2P) (or other distributed) networkenvironment. The machine may be a personal computer (PC), a tablet PC, aset-top box (STB), a personal digital assistant (PDA), a mobiletelephone, a web appliance, a network router, switch or bridge, or anymachine capable of executing instructions (sequential or otherwise) thatspecify actions to be taken by that machine. Further, while only asingle machine is illustrated, the term “machine” shall also be taken toinclude any collection of machines that individually or jointly executea set (or multiple sets) of instructions to perform any one or more ofthe methodologies discussed herein, such as cloud computing, software asa service (SaaS), other computer cluster configurations.

Examples, as described herein, may include, or may operate by, logic ora number of components, or mechanisms. Circuit sets are a collection ofcircuits implemented in tangible entities that include hardware (e.g.,simple circuits, gates, logic, etc.). Circuit set membership may beflexible over time and underlying hardware variability. Circuit setsinclude members that may, alone or in combination, perform specifiedoperations when operating. In an example, hardware of the circuit setmay be immutably designed to carry out a specific operation (e.g.,hardwired). In an example, the hardware of the circuit set may includevariably connected physical components (e.g., execution units,transistors, simple circuits, etc.) including a computer readable mediumphysically modified (e.g., magnetically, electrically, moveableplacement of invariant massed particles, etc.) to encode instructions ofthe specific operation. In connecting the physical components, theunderlying electrical properties of a hardware constituent are changed,for example, from an insulator to a conductor or vice versa. Theinstructions can enable embedded hardware (e.g., the execution units ora loading mechanism) to create members of the circuit set in hardwarevia the variable connections to carry out portions of the specificoperation when in operation. Accordingly, the computer readable mediumis communicatively coupled to the other components of the circuit setmember when the device is operating. In an example, any of the physicalcomponents may be used in more than one member of more than one circuitset. For example, under operation, execution units may be used in afirst circuit of a first circuit set at one point in time and reused bya second circuit in the first circuit set, or by a third circuit in asecond circuit set at a different time.

Particular implementations of the systems and methods described hereinmay involve use of a machine (e.g., computer system) that may include ahardware processor (e.g., a central processing unit (CPU), a graphicsprocessing unit (GPU), a hardware processor core, or any combinationthereof), a main memory and a static memory, some or all of which maycommunicate with each other via an interlink (e.g., bus). The machinemay further include a display unit, an alphanumeric input device (e.g.,a keyboard), and a user interface (UI) navigation device (e.g., amouse). In an example, the display unit, input device and UI navigationdevice may be a touch screen display. The machine may additionallyinclude a storage device (e.g., drive unit), a signal generation device(e.g., a speaker), a network interface device, and one or more sensors,such as a global positioning system (GPS) sensor, compass,accelerometer, or other sensor. The machine may include an outputcontroller, such as a serial (e.g., universal serial bus (USB),parallel, or other wired or wireless (e.g., infrared (IR), near fieldcommunication (NFC), etc.) connection to communicate or control one ormore peripheral devices (e.g., a printer, card reader, etc.).

The storage device may include a machine readable medium on which isstored one or more sets of data structures or instructions (e.g.,software) embodying or utilized by any one or more of the techniques orfunctions described herein. The instructions may also reside, completelyor at least partially, within the main memory, within static memory, orwithin the hardware processor during execution thereof by the machine.In an example, one or any combination of the hardware processor, themain memory, the static memory, or the storage device may constitutemachine readable media.

While the machine readable medium can include a single medium, the term“machine readable medium” may include a single medium or multiple media(e.g., a centralized or distributed database, and/or associated cachesand servers) configured to store the one or more instructions.

The term “machine readable medium” may include any medium that iscapable of storing, encoding, or carrying instructions for execution bythe machine and that cause the machine to perform any one or more of thetechniques of the present disclosure, or that is capable of storing,encoding or carrying data structures used by or associated with suchinstructions. Non-limiting machine readable medium examples may includesolid-state memories, and optical and magnetic media. In an example, amassed machine readable medium comprises a machine readable medium witha plurality of particles having invariant (e.g., rest) mass.Accordingly, massed machine-readable media are not transitorypropagating signals. Specific examples of massed machine readable mediamay include: non-volatile memory, such as semiconductor memory devices(e.g., Electrically Programmable Read-Only Memory (EPROM), ElectricallyErasable Programmable Read-Only Memory (EEPROM)) and flash memorydevices; magnetic disks, such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM and DVD-ROM disks.

The instructions may further be transmitted or received over acommunications network using a transmission medium via the networkinterface device utilizing any one of a number of transfer protocols(e.g., frame relay, internet protocol (IP), transmission controlprotocol (TCP), user datagram protocol (UDP), hypertext transferprotocol (HTTP), etc.). Example communication networks may include alocal area network (LAN), a wide area network (WAN), a packet datanetwork (e.g., the Internet), mobile telephone networks (e.g., cellularnetworks), Plain Old Telephone (POTS) networks, and wireless datanetworks (e.g., Institute of Electrical and Electronics Engineers (IEEE)802.11 family of standards known as Wi-Fi®, IEEE 802.16 family ofstandards known as WiMax®), IEEE 802.15.4 family of standards,peer-to-peer (P2P) networks, among others. In an example, the networkinterface device may include one or more physical jacks (e.g., Ethernet,coaxial, or phone jacks) or one or more antennas to connect to thecommunications network. In an example, the network interface device mayinclude a plurality of antennas to wirelessly communicate using at leastone of single-input multiple-output (SIMO), multiple-inputmultiple-output (MIMO), or multiple-input single-output (MISO)techniques. The term “transmission medium” shall be taken to include anyintangible medium that is capable of storing, encoding or carryinginstructions for execution by the machine, and includes digital oranalog communications signals or other intangible medium to facilitatecommunication of such software.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments can be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The claimed invention is:
 1. A system comprising: a linear digitalpredistortion (DPD) circuit programmed to perform operations comprising:receiving a complex baseband signal; and generating a pre-distortedsignal linear component based at least in part on the complex basebandsignal; a nonlinear DPD circuit programmed to perform, operationscomprising: receiving the complex baseband signal; determining areal-mode pre-distorted nonlinear component based at least in part onthe complex baseband signal; and generating a pre-distorted signalnonlinear component based at least in part on the real-modepre-distorted nonlinear component; and a mixer circuit programmed togenerate a pre-distorted signal based at least in part on thepre-distorted signal linear component and the pre-distorted signalnonlinear component.
 2. The system of claim 1, further comprising acomplex-to-real circuit programmed to determine the real-modepre-distorted nonlinear component.
 3. The system of claim 1, furthercomprising: an interpolator circuit to up-sample the complex basebandsignal to generate an up-sampled complex baseband signal; and anup-converter circuit to translate the up-sampled complex baseband signalto a carrier frequency to generate an up-converted complex signal,wherein the mixer circuit is also to generate the pre-distorted signalbased at least in part on the up-converted complex signal.
 4. The systemof claim 1, wherein the nonlinear DPD circuit is also programmed toperform operations comprising: determining a real component based atleast in part on the complex baseband signal; determining a first valuefor the real component at a first time; and selecting from a firstnonlinear look up table (LUT) a first LUT value based at least in parton the first value for the real component, wherein the first LUT valueis a based at least in part on a sum of terms omitting a first orderterm.
 5. The system of claim 4, wherein the nonlinear DPD circuit isalso programmed to perform operations comprising: selecting from asecond LUT a second LUT value based at least in part on a second valuefor the real component at a second time before the first time; anddetermining a sum of the first LUT value and the second LUT value,wherein the pre-distorted signal nonlinear component is based at leastin part on the stum.
 6. The system of claim 1, wherein the nonlinear DPDcircuit is also programmed to perform operations comprising: determininga real component based at least in part on the complex baseband signal;determining a first value for the real component at a first time;determining an I^(th) order power of the first value; applying an I^(th)order tap coefficient to the I^(th) order power of the first value togenerate an I^(th) order tap value; determining a J^(th) order power ofthe first value; and applying a J^(th) order tap coefficient to theJ^(th) order power of the first value to generate a J^(th) order tapvalue, wherein the pre-distorted signal nonlinear component is based atleast in part a sum of the I^(th) order tap value and the J^(th) ordertap value.
 7. The system of claim 1, wherein the nonlinear DPD circuitis further programmed to perform operations comprising converting thereal-mode pre-distorted nonlinear component to a complex pre-distortednonlinear component.
 8. The system of claim 7, wherein the nonlinear DPDcircuit is further programmed to perform operations comprisingtranslating the complex pre-distorted nonlinear component to baseband.9. The system of claim 1, wherein the linear DPD circuit is furtherprogrammed to perform operations comprising applying a scalar gain stageto the complex baseband signal.
 10. The system of claim 1, furthercomprising a complex time align circuit programmed to align thepre-distorted linear component and the pre-distorted nonlinearcomponent.
 11. The system of claim 1, further comprising: a tiltreference filter circuit programmed to perform operations comprisingapplying a highpass frequency gain characteristic to a real component ofthe complex baseband signal to generate a tilt reference real component,wherein the nonlinear DPD circuit is also programmed to performoperations comprising determining a tilt reference real pre-distortedsignal nonlinear component based at least in part on the tilt referencereal component.
 12. The system of claim 1, wherein the nonlinear DPDcircuit is further programmed to perform operations comprisinggenerating a real pre-distorted signal nonlinear component at a carrierfrequency, further comprising: a complex decimation filter circuitprogrammed to perform operations comprising applying a passband to thereal pre-distorted signal nonlinear component to generate a quadraturepre-distorted signal nonlinear component, wherein the passband has abandwidth matching a bandwidth of the complex baseband signal centeredat the carrier frequency; and a demodulation circuit programmed toperform operations comprising demodulating the quadrature pre-distortedsignal nonlinear component to baseband.
 13. A method comprising:receiving a complex baseband signal at a linear DPD circuit; generating,with the linear DPD circuit, a pre-distorted signal linear componentbased at least in part on the complex baseband signal; receiving thecomplex baseband signal at a nonlinear DPD circuit; determining areal-mode pre-distorted nonlinear component based at least in part onthe complex baseband signal; generating, with the nonlinear DPD circuit,a pre-distorted signal nonlinear component based at least in part on thereal-mode pre-distorted nonlinear component; and generating apre-distorted signal based at least in part on the pre-distorted signallinear component and the pre-distorted signal nonlinear component. 14.The method of claim 13, further comprising: up-sampling the complexbaseband signal to generate an up-sampled complex baseband signal; andtranslating the up-sampled complex baseband signal to a carrierfrequency to generate an up-converted complex signal, wherein generatingthe pre-distorted signal is also based at least in part on theup-converted complex signal.
 15. The method of claim 13, furthercomprising: determining a real component based at least in part on thecomplex baseband signal; determining a first value for the realcomponent at a first time; and selecting from a first nonlinear look uptable (LUT) a first LUT value based at least in part on the first valuefor the real component, wherein the first LUT value is a based at leastin part on a sum of terms omitting a first order term.
 16. The method ofclaim 15, further comprising: selecting from a second LUT a second LUTvalue based at least in part on a second value for the real component ata second time before the first time; and determining a sum of the firstLUT value and the second LUT value, wherein the pre-distorted signalnonlinear component is based at least in part on the sum.
 17. The methodof claim 13, further comprising: determining a real component based atleast in part on the complex baseband signal; determining a first valuefor the real component at a first time; determining an I^(th) orderpower of the first value; applying an I^(th) order tap coefficient tothe order power of the first value to generate an I^(th) order tapvalue; determining a J^(th) order power of the first value; and applyinga J^(th) order tap coefficient to the J^(th) order power of the firstvalue to generate a J^(th) order tap value, wherein the pre-distortedsignal nonlinear component is based at least in part a. sum of theI^(th) order tap value and the J^(th) order tap value.
 18. The method ofclaim 13, further comprising: determining a real-mode pre-distortednonlinear component; and converting the real-mode pre-distortednonlinear component to a complex pre-distorted nonlinear component. 19.The method of claim 13, further comprising applying a scalar gain stageto the complex baseband signal.
 20. The method of claim 13, furthercomprising a complex time align circuit programmed to align thepre-distorted linear component and the pre-distorted nonlinearcomponent.
 21. The method of claim 13, further comprising: applying ahighpass frequency gain characteristic to a real component of thecomplex baseband signal to generate a tilt reference real component;determining a tilt reference real pre-distorted signal nonlinearcomponent based at least in part on the tilt reference real component;and applying a tilt equalizer to the tilt reference real pre-distortedsignal to generate the real pre-distorted signal nonlinear component.22. The method of claim 13, further comprising: generating a realpre-distorted signal nonlinear component at a carrier frequency;applying a passband to the real pre-distorted signal nonlinear componentto generate a quadrature pre-distorted signal nonlinear component,wherein the passband has a bandwidth matching a bandwidth of the complexbaseband signal centered at the carrier frequency; and demodulating thequadrature pre-distorted signal nonlinear component to baseband.
 23. Adigital predistortion (DPD) apparatus, comprising: means receiving acomplex baseband signal; means for generating a pre-distorted signallinear component based at least in part on the complex baseband signal;means for receiving the complex baseband signal at a nonlinear DPDcircuit; means for determining a real-mode pre-distorted nonlinearcomponent based at least in part on the complex baseband signal; meansfor generating a pre-distorted signal nonlinear component based at leastin part on the real-mode pre-distorted nonlinear component; and meansfor generating a pre-distorted signal based at least in part on thepie-distorted signal linear component and the pre-distorted signalnonlinear component.